1. Field of the Invention
The present invention relates to a gradient coil for a magnetic resonance imaging apparatus. In particular, the present invention relates to a gradient coil for a magnetic resonance imaging apparatus, which is capable of suppressing magnetic interactions with peripheral structures, and to a magnetic resonance imaging apparatus using the gradient coil.
2. Description of the Related Art
In a diagnosis using a nuclear magnetic resonance, the intensity of a magnetic field is associated with an area to be diagnosed. It is demanded that the intensity of a magnetic field generated by a magnet system vary within a range of about one millionth of the intensity of the magnetic field. Magnetic fields generated by the magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) are mainly divided into the following three types.    (1) Static magnetic field which is constant in terms of time and space. The intensity of the static magnetic field is typically 0.1 to several Tesla or more. The static magnetic field varies by several ppm in a space (in general, space of a sphere having a diameter of 30 to 40 cm or elliptic shape, hereinafter referred to as an imaging area) in which imaging is performed.    (2) Gradient magnetic field that varies at a time constant of one second or less and is spatially inclined.    (3) Magnetic field generated by an electromagnetic wave with a high frequency (several MHz or higher) for a nuclear magnetic resonance.
The static magnetic field described in (1), which is constant in terms of time and space, is typically generated by a permanent magnet or coil conducting a current. Magnetizing a magnetic body arranged at an appropriate place makes the static magnetic field more uniform.
The gradient magnetic field described in (2) varies in terms of time in the order of several Hz to 100 kHz. The gradient magnetic field is generated by a coil (hereinafter referred to as a gradient coil) in which a current varying in terms of time flows. By adding the gradient magnetic field, the frequency and the position of a nuclear magnetic resonance which is added in the type (3) are associated with each other. In addition, it is necessary that a magnetic field that is three-dimensionally inclined be provided for the gradient magnetic field. For the three dimensions, three types of combinations of coils are used.
FIG. 2 is a diagram showing the arrangement including magnetic pole pieces generating the abovementioned conventional magnetic fields and an apparatus which is a vertical magnetic field type MRI apparatus with an open type static magnetic field magnet. In FIG. 2, reference numeral 1 denotes a gradient coil; 3, a magnetic pole piece; 4, a connecting pole; 5, imaging area; and 11, a gradient magnetic field. The upper and lower magnetic pole pieces 3 are supported by the connecting poles 4. The lower diagram of FIG. 2 is a cross sectional view showing the MRI apparatus including the central axis thereof. The imaging area 5 is present between the magnetic pole pieces 3. The upper and lower magnetic pole pieces 3 each have a recessed portion on the side of the imaging area 5. The gradient coil 1 is arranged in each of the recessed portions. A static magnetic field 10 having a high intensity is generated in a direction of a large arrow shaded with dots by coils or permanent magnets. The coils or permanent magnets are arranged in the two magnetic pole pieces 3, which are means for forming a static magnetic field. The static magnetic field is in the order of 0.1 to 10 Tesla.
Upward and downward solid line arrows indicate magnetic lines of force of the gradient magnetic field 11. The upward and downward arrows drawn from an intermediate surface between the upper and lower magnetic pole pieces 3 schematically indicate the direction and size of the gradient magnetic field 11. The farther the gradient magnetic field 11 is from each of the original points of the magnetic lines of force, the higher the intensity of the gradient magnetic field 11 is. The sign of the gradient magnetic field on the upper side of the imaging area 5 is different from the sign of the gradient magnetic field on the lower side thereof. A central axis vertically extending through the center of the imaging area 5 is taken as Z axis. X axis and Y axis are located on a horizontal plane perpendicular to Z axis. The arrows indicate a distribution of the gradient magnetic field generated by an X axis gradient magnetic field coil, which varies in intensity in the X axis direction (or generated by a Y axis gradient magnetic field coil, which varies in intensity in the Y axis direction). The X axis gradient magnetic field coil (or the Y axis gradient magnetic field coil) is the gradient coil 1. The gradient magnetic field varies in intensity and direction depending on the location. The gradient magnetic field has an intensity of about ±100 Gauss and varies in pulse from the order of 0.1 milliseconds to the order of 1 second.
FIG. 4 is a diagram showing a coil pattern of the X axis gradient magnetic field coil (or the Y axis gradient magnetic field coil) that generates a magnetic field. The magnetic field has an intensity showing an inclination in a direction parallel to the surface of the gradient coil. Conductive windings 6 corresponding to a current pattern of each of the coils form a spiral pattern having a single central point (Toshie Takeuchi, et al., “Design of Flat Type Self Shielded gradient coil”, The Institute of Electrical Engineers of Japan, 1998, Vol. 118, No. 3, pp. 287-292). In FIG. 4, a crossover 7 between winding loops is not shown (also not shown in the other drawings). The coil present on the side of the imaging area 5 is a primary gradient coil 20. The coil positioned on the side opposite to the imaging area 5 is a shielded gradient coil 21.
FIG. 3 is a schematic diagram showing magnetic lines of force generated by the gradient coils. The gradient coils are provided in the arrangement including the magnetic pole pieces 3 and the apparatus as shown in FIG. 2. The magnetic lines of force shown in FIG. 3 are present at the recessed portion of the magnetic pole pieces 3. The arrows indicated with solid lines and the arrows indicated with dotted lines indicate the magnetic lines of force of the magnetic field generated by the gradient coils. In FIG. 3, superconductive coils 8 are also shown. Each of the superconductive coils 8 generates a static magnetic field in the imaging area 5 and is arranged annularly around Z axis. The arrows drawn from an intermediate surface between the upper and lower magnetic pole pieces 3 qualitatively indicate the directions and sizes of the magnetic fields generated by the gradient coils. The arrows passing through the gradient coils each indicate the flows of the magnetic lines of force.
The gradient magnetic field 11 indicated by the solid lines passes through the imaging area 5 and the periphery thereof and moves in the gradient coils. After that, the gradient magnetic field 11 returns to the position which is on the side opposite to the position represented by the original coordinates in the imaging area 5. In this case, the gradient magnetic field 11 is oppositely directed. The gradient magnetic lines of force return within the gradient coils in such a manner as to prevent the magnetic field in a shielded gradient coil from leaking to the sides of the magnetic pole pieces 3. On the other hand, the magnetic lines of force (indicated by dotted lines) of a leak magnetic field 12, which are generated by the gradient coil, pass through areas that are away from the imaging area 5. After that, a part of the magnetic lines of force passes through the inside of the magnetic pole pieces 3.
FIG. 4 is a diagram showing conventional gradient coils. For the conventional gradient coils, a current flows in an area of each surface of the coils in a direction opposite to a current flowing at the central area, the area of each surface of the coils having a large diameter and being far from the imaging area 5. In this coil system, although a magnetic field is not generated in a shielded gradient coil on the side opposite to a primary gradient coil, a magnetic field leaks to the outside of the coil system. Such a magnetic field is indicated by the dotted arrows as the magnetic field 12 shown in FIG. 3.
In such gradient coils, when a conductive structural member such as a magnetic polar surface 9 is present in an area having a large diameter, an eddy current is generated on the magnetic polar surface 9. A magnetic field caused by the eddy current disturbs the gradient magnetic field 11 in terms of time and space. This makes it difficult to obtain a clear image through magnetic resonance imaging.
A magnetomotive force source such as the superconductive coil 8 is present in the magnetic pole pieces 3. If the gradient magnetic field leaks into those areas and interferes with the magnetomotive force source, a vibration of an internal structural member such as a coil vibration may occur. As a result, the magnetic field may be oscillated. This makes it difficult to obtain a clear image through magnetic resonance imaging.
The gradient coils are required to generate a desirably accurate gradient magnetic field and have the following:
a property to provide a high intensity per unit of electric current (high efficiency);
a high response property (low inductance);
a property to suppress the generation of an eddy current and to reduce a dynamic error magnetic field, and
a small interaction with a magnet that generates a static magnetic field or a structural member of the magnet due to an electromagnetic force.
The above properties can be realized by reducing a magnetic field that is generated in an area other than a necessary area to a minimum possible extent. Specifically, if a magnetic field is generated in an unnecessary area, magnetic energy is required in the unnecessary area, resulting in an increase in inductance. In addition, if a structural member is present in the unnecessary area, an eddy current may be generated, or an electromagnetic interaction may occur. Furthermore, the intensity of a necessary magnetic field is relatively reduced, resulting in a reduction in efficiency. Therefore, when an unnecessary magnetic field present in an area other than the imaging area 5 is reduced, the abovementioned properties can be improved.